1. Field of the Invention
The present invention relates to semiconductor display devices with thin film transistors. More particularly, the invention relates to a technology to manufacture a plurality of thin film transistors free of characteristic variations through the use of a linear laser beam.
2. Description of the Prior Art
In recent years studies have been eagerly made in order to decrease the process temperature in the manufacture of thin film transistors (hereinafter referred to as TFTs). The major reason for this is due to a necessity that a semiconductor device is to be formed on an insulator substrate of such as a glass that is low in cost but high in processability. The temperature decrease in the process to manufacture a semiconductor device is also demanded from a viewpoint of put forward with device scale down and multilevel structure.
The manufacture of a high performance semiconductor device requires a process to crystallize an amorphous ingredient or amorphous semiconductor material contained in a semiconductor material. Meanwhile, there might require a process to restore deteriorated crystallinity in a semiconductor material whose property is crystalline in nature but deteriorated by ion irradiation, or a process to improve crystallinity furthermore. Conventionally, thermal anneal has been utilized for such purposes. Where silicon has been employed as a semiconductor material, anneal has been conducted at temperatures of 600.degree. C. to 1100.degree. C. for 1 to 48 hours or longer in order to cause amorphous crystallization, crystallinity restoration, crystallinity improvement and so on.
In the thermal anneal for the above purposes, the high the process temperature the shorter the required process time becomes. However, there is almost no preferred effect at a temperatures of 500.degree. C. or below. From a viewpoint of temperature decrease, it has been conventionally considered that the process using thermal anneal has to be substituted by another means. Particularly it has been considered that, where a glass is used as a substrate, the glass substrate has a heat resisting temperature of around 600.degree. C. and accordingly requires such a means to be effected at a temperature below this that corresponds to the conventional thermal anneal.
In recent, attentions have been drawn to techniques attempted to irradiate laser light onto a semiconductor material in order to perform various types of anneal. The thermal anneal with laser light irradiation has an advantage that there is no necessity to exposing the entire substrate to a high temperature because of its capability to give high energy equivalent to that of thermal anneal to a desired limited point.
There are, roughly, two proposals as a method to irradiate laser light.
The first method uses a continuous oscillation laser such as an argon ion laser, in order to irradiate a spot-formed beam to a semiconductor material. This method utilizes a difference of in-beam energy distribution and beam movement to cause fusion in a semiconductor material and then moderate solidification thereby crystallizing the semiconductor material.
The second method uses a pulse oscillation laser alike an excimer laser to irradiate a great energy laser pulse to a semiconductor material, wherein upon laser irradiation the semiconductor material instantaneously fuses and solidifies thus utilizing crystal growth proceeding.
The first method involves a problem of taking a long time to perform the process. This is because the continuous oscillation laser is limited in its maximum energy and the beam spot size is at most on a order of a millimeter square. In contrast to this, the second method the laser has a great maximum energy by which a spot as large as several centimeter square or greater can be used to enhance mass productivity.
However, where using a beam in a usual square or rectangular form, there is a necessity of moving the beam in vertical and horizontal directions in order to process an entire substrate with a wide area. Thus a problem has still been left in respect of productivity (throughput).
To cope with this, the throughput can be largely improved by adopting a method wherein the beam shape is changed into a linear form having a beam width greater than that of a substrate to be processed in order to implement scanning the beam over the substrate relative thereto. The scan herein refers to linear laser irradiation with slightly shifting while overlapping.
However, where applying the above technique using linear pulse laser irradiation with overlap while slight shifting, linear fringes naturally occur on a surface of a laser-irradiated semiconductor material. These fringes has a great adverse effect upon characteristics of a device having been formed or to be formed on the surface of the semiconductor material. In particular, a serious problem will be encountered when a plurality of devices are to be formed on the substrate with an even characteristic on a one-by-one device basis. In such a case, the fringe pattern has variation in characteristic occurring between the fringes despite each fringe is homogeneous in characteristic.
In also the anneal method using a linear laser light, a problem rises in respect of evenness by the effect of irradiation. High evenness herein refers to the ability to provide an even device characteristic regardless of a device forming position on the substrate. The improvement in evenness means to make homogeneous the crystallinity of a semiconductor material. The following attempts have being made in order to raise the evenness.
It is known that the evenness is improved by preparatorily irradiating (hereinafter referred to as preparatory irradiation) a pulse laser light with a weaker intensity prior to irradiating a stronger pulse laser light (hereinafter referred to as main irradiation) in order to relax unevenness due to laser irradiation effects. This is extremely effective and improve a semiconductor device circuit characteristic to a significant extent with variation suppressed.
The reason why the preparatory irradiation effective for film homogeneousness is that a semiconductor material film containing an amorphous portion as stated before has such a property of laser energy absorption ratio that is significantly different from that of a polysilicon film or single crystal film. That is, two stage irradiation acts to crystallize, in a first irradiation, amorphous portions remained in the film and, at a second irradiation, accelerates entire crystallization. The moderate crystallization as this serves to suppress to a certain extent fringes from occurring on the semiconductor material due to linear laser irradiation. This attempt considerably improves the laser light irradiation effect and the fringes as observed become comparatively modest.
However, in the case that a multiplicity (on the order of several millions to several tens of millions) of thin film transistors are required to form on a glass substrate as in an active matrix semiconductor display device, e.g., a liquid crystal display device, even the laser irradiation method with two stage irradiation is unsatisfactory in respect of its evenness effect.
Here, a schematic configuration diagram of a conventional active matrix liquid crystal display is shown in FIG. 8. In FIG. 8, 801 is a shift register on a source signal line side, 802 and 803 are buffers (inverters), 804 is an analog switches, 805 is a video signal line, 806 is a source signal line, 807 is a shift register on a gate signal line side, 808 is a buffer (inverter), 809 is a gate signal line, 810 is a pixel TFTs and 811 is a liquid crystal. Also, FIGS. 9(a) and 9(b) demonstrate circuit diagrams for the buffers (inverters) 802, 803 and 808 and the analog switch 804.
In the buffer of FIG. 9(a), IN represents that a timing signal is inputted from the shift register while OUT denotes outputting an inverted signal thereof. Also, Vdd is a constant power voltage. In the analog switch of FIG. 9(b), IN represents inputting of a signal from the buffer while INb inputting of an inverted signal thereof. VIDEO IN is inputted with a video signal from the video signal line, and VIDEO OUT outputs a video signal therefrom.
Reference is now made to FIG. 8. The shift register 801 on the source signal line side sequentially supplies timing signals to the buffer 802. The timing signals are amplified by the buffer 802 to control the opening and closing of the analog switch 804. An image signal is introduced from the video signal line 805 through the analog switch 804, and is then supplied through the source signal line 806 to a corresponding pixel TFT 810. Based on the timing signals sequentially supplied from the shift register 807 of the gate signal line side, the buffer 808 supplies scan signals through the gate signal line 809 to a corresponding pixel TFT 810. Accordingly, a pixel TFT 810 supplied (i.e., selected by) with the scan signal and image signal supplies a voltage to the liquid crystal 811 through a pixel electrode connected to a drain region thereof, thereby driving the liquid crystal. At this time, the transmission light through the liquid crystal varies in intensity, thereby providing images.
The factor to cause deterioration particularly in image quality (unevenness in display) for the active matrix liquid crystal display device includes variation in characteristic of the analog witch or the buffer.
This is due to the fact that there encounters an inevitable increase in load capacitance of the source signal line and the gate signal line in an attempt to achieve an increase in precision and resolution for the active matrix liquid crystal display. To drive the source signal line having an increased load capacitance requires a increased capacitance of an analog switch. Moreover, the operation of an analog switch with an increased capacitance requires a buffer with an increased capacitance. Where forming analog switches or buffers having an increased capacitance by thin film transistors (TFTs), the TFTs have to be made with increased capacitance, or increased channel width. The TFTs, if great in channel width, have variation in crystallinity within the device with a result that the TFTs have variation in their threshold voltage. It is therefore natural that variation is introduced in the analog switch or analog buffer formed by a plurality of TFTs. This results in an existence of analog switches or analog buffers varied in characteristic by a source signal line. The variation in characteristic leads to variation in application voltage to the liquid crystal. This is reflected by unevenness in display entirely in the display device.
In the meanwhile, if the size (channel width) of the TFT is excessively large, the TFT at its end does not function as a channel despite it at a center functions as a channel, resulting in a possibility to accelerate deterioration.
Furthermore, where the TFT is great in size, the TFT has increased self heat generation, leading to variation in threshold value or deterioration.
This also requires increase in capacitance for the buffers to drive the gate signal line, leading to variation in characteristic, deterioration, self heat generation and so on similarly to the case of the source signal line.
Consequently, the TFT unevenness induces deterioration in image quality (unevenness in display) particularly for the active matrix liquid crystal display device, resulting in a major factor to reduce product yield.